A capacitance video recording and playback system has been disclosed in U.S. Pat. Nos. 3,842,194, 3,842,217 and 3,909,517 to Clemens, incorporated herein by reference. According to this system, vinyl disc replicas can be prepared having geometric variations in a spiral groove in the disc surface which correspond to capacitance variations representative of video signals. The discs are coated first with a thin conductive metal layer and then with a dielectric layer. A metal stylus completes the capacitor and, during playback, rides upon the dielectric coating and detects dimensional variations in the groove. These variations are reconstituted in electrical signal form and converted back to video information suitable for display by a television receiver.
Since the relief pattern and the groove are of very small dimensions, e.g., there are 5000-10,000 grooves per inch (12,700-25,400 grooves per centimeter), much research and expense have been required to learn how to put down metal and dielectric layers which conform to the relief pattern, are thin enough so that they do not fill the grooves and yet are thick enough so as to form a coherent, continuous layer.
It will be apparent that if the metal and dielectric layers could be eliminated, with no substantial loss of signal or increase in noise level on playback of the disc, it would be a great advance as far as cost savings in manufacturing are concerned.
Fox et al in copending U.S. application entitled "Non-Coated Video Disc Replicas", filed July 27, 1977, Ser. No. 818,279, have disclosed non-coated, conductive video disc replicas made by molding a plastic material containing sufficient finely divided conductive particles so that the material has a bulk resistivity below about 500 ohm-centimeters at 900 megahertz, the playback frequency. Excellent video disc replicas can be compression molded using vinyl chloride homopolymers and copolymers containing a stabilizer, a lubricant and, optionally, a processing aid containing about 12-20 percent by weight of a conductive carbon black having a low bulk density.
However, when it was desired to scale up the process to commercial quantities, difficulties were encountered with the existing molding composition. When mixing large quantities of molding compound, high temperatures are generated due to the high shear and length of time required for good dispersion of the ingredients, particularly large amounts of conductive particles. Excessive temperatures lead to decomposition and high volatiles generation, as well as staining and excessive bleed out of the additives. Further, the melt viscosity of the molding composition was too high for compression molding on a large scale. In addition, the video disc replicas were rather brittle and tended to warp on storage under high temperature, high humidity conditions.